This invention relates to the field of environmental scanning electron microscopes, and more particularly, to an environmental scanning electron microscope which achieves standard SEN resolution performance in a gaseous environment.
As background, the advantages of an environmental scanning electron microscope over standard scanning electron microscopes (SEN) lie in its ability to produce high-resolution electron images of moist or non-conductive specimen (e.g., biological materials, plastics, ceramics, fibers) which are extremely difficult to image in the usual vacuum environment of the SEN. The environmental scanning electron microscope allows the specimen to be maintained in its "natural" state without subjecting it to the distortions caused by drying, freezing, or vacuum coating normally required for high-vacuum electron beam observation. Also, the relatively high gas pressure easily tolerated in the environmental scanning electron microscope specimen chamber acts effectively to dissipate the surface charge that would normally build up on a non-conductive specimen, blocking high quality image acquisition. The environmental scanning electron microscope also permits direct, real-time observation of liquid transport, chemical reaction, solution, hydration, crystallization, and other processes occurring at relatively high vapor pressures far above those can be permitted in the normal SEM specimen chamber.
As stated in U.S. Pat. No. 4,992,662, the original concept of an environmental scanning electron microscope, as suggested in U.S. Pat. No. 4,596,928, was to maintain the specimen chamber in a gaseous environment such that the gaseous environment acted as a conditioning medium in order to maintain the specimen in a liquid or natural state. In addition, the utilization of the gaseous environment of the specimen chamber as a medium for amplification of the secondary electron signals is described in U.S. Pat. No. 4,785,182.
In the environmental SEM of U.S. Pat. No. 4,824,006, electron beam observation of unprepared, full-sized specimens at high vacuum pressures was made possible due to the combination of pressure control and signal detection means, housed entirely within the magnetic objective lens of the electron beam column. The environmental SEM design of U.S. Pat. No. 4,823,006 satisfied the simultaneous requirements for pressure control, electron beam focusing, and signal amplification, while providing no practical limitations on specimen handling or microscopic resolving power.
A limitation of the environmental SEM of U.S. Pat. No. 4,824,006, however, is that the field-of-view of the sample is limited by the final pressure limiting aperture. The pressure limiting aperture is necessary to prevent the gas from the specimen chamber affecting the operation of the electron column and absorbing the primary electron beam. This pressure limiting aperture is typically 0.5 millimeters in diameter. This also limits the field-of-view to about a 0.5 millimeter diameter. This limited field-of-view makes it difficult for the operator to know where he or she is on the sample and the location of the desired area of examination.
It is therefore desirable to provide an environmental scanning electron microscope having an optical window system which allows the operator to switch between the normal environmental scanning electron microscope electron image (limited to approximately 0.5 millimeters in diameter) to an optical light view of the sample that covers a field-of-view of about 7 to 10 mm which is comparable with typical field-of-view capabilities of an SEM.
U.S. Pat. No. 4,897,545 describes a more complex detector arrangement than that disclosed in U.S. Pat. No. 4,824,006 which allows for the detection of other signals (i.e., backscattered electrons) in the gas, utilizing a set of differently biased electrodes. U.S. Pat. No. 4,897,545 discusses that various electrodes can be used to collect different signals from the specimen chamber, but fails to discuss how the signal from one electrode might be optimized by a suitable bias on another electrode. In addition, in U.S. Pat. No. 4,897,545, there is no mention of the concept of using an electrode as a collector of unwanted signals or using an electrode to reduce signal noise produced by the primary beam. It would therefore be desirable to provide an environmental SEM which improves secondary electron detection by reducing the backscattered electron component of the signal and the signal noise produced by the primary beam.
In comparison to the environmental SEMs of U.S. Pat. Nos. 4,824,006 and 4,897,545, it has also been found desirable to provide an environmental SEM which optimizes signal amplification following detection of the desired secondary electrons such that the detector noise is reduced below the noise in the signal itself, while still maintaining an overall signal bandwidth that is suitable for setting up the image.
Most modern SEMs utilize a method to enable an electron beam to be scanned over the sample in order to construct an image of the sample, or to collect other information from the sample, such as X-ray data. These methods allow a large part of the sample to be scanned with acceptably low geometric distortion of the image of the sample. In a typical SEM, the electron gun generates a relatively large (20 micrometer) source of electrons which must be demagnified to the small size required to image small details on the sample. This de-magnification is typically produced by three electron lenses in the optical vacuum column. Only a small proportion of the electrons reach the sample as the final beam is conveniently referred to as a "pencil beam" (see reference numeral 2 in FIG. 1).
As is shown in FIG. 2, in order to scan the primary beam along the length of the vacuum column, scanning coils, such as 3 and 4, are positioned in the objective lens assembly 5. If a current is passed through the two sets of scanning coils 3 and 4, the pencil beam will be deflected as shown in FIG. 2.
In the previous environmental SEM of FIG. 3, two pressure limiting apertures 6 and 7 allow high pressure in the specimen chamber 8 (e.g., approximately 5 Torr) while maintaining a high vacuum (e.g., approximately 0.0001 Torr) in the region of the scanning coils 9a and 9b. As these pressure limiting apertures are normally 0.3 mm to 0.5 mm in diameter, they limit the field-of-view of the specimen, as aforementioned, to about 0.5 mm in diameter. In contrast, a typical SEM will allow at least ten times this amount under comparable working conditions. It is therefore desirable to provide a scanning coil deflection system for an environmental scanning electron microscope having at least two pressure limiting apertures which has an enhanced field-of-view.
Moreover, in the prior environmental SEM, such as the environmental SEM of U.S. Pat. No. 4,824,006, the take-off angle of the X-ray detector (EDX detector) is not comparable to the take-off angle of the EDX detectors of conventional SEMs. As is shown in FIG. 4, in a conventional SEM, the conical base 201 of the objective lens 202 allows the energy dispersive X-ray detector (EDX) 203 to collect X-rays emanating from the surface of the sample at a take-off angle of approximately 30%, without the bulk of the EDX detector intruding into the sample space.
In environmental SEMs, however, as is shown in FIG. 5, the objective lens 204 with a flat lower pole piece 205 and the gas manifolds 206 and 207 (used to withdraw the gas that passes through the pressure limiting aperture) are at the same level. Since the working distance between the specimen and the data acquisition devices in an environmental SEM must be kept as short as possible, the space that the EDX detector can occupy in the specimen chamber is restricted. Thus, in order to limit this working distance, the design of the environmental SEM of FIG. 5 limits the take-off angle of the EDX detector 208 in the specimen chamber to approximately 20.degree.. It is therefore desirable to provide an environmental SEM which provides for a take-off angle of the EDX detector which is comparable with the take-off angle of EDX detectors in conventional SEMs.